Moving to iMac from Macbook for UI/UX Design (Need advice)

[maxmaut]

Hey guys.

I am switching from Macbook to iMac and need advice.

I design iOS mobile apps.


What I use now:
15 inch, rMBP Early 2013,
2.7GHz i7,
16 gigs of RAM,
GeForce GT 650M 1Gb,
768 Gb SSD.

I work in Adobe Creative Suite apps.

What's wrong about it:
When files get big (for big apps) Adobe Apps lag.
I use scaling to stuff more UI elements into my 15'' retina. Stuff gets small and my head and eyes want to be closer to the screen, so I am constantly crooked and never sit up straight.
Hooking up external monitor while doing Skype video call and screensharing slows down the computer drastically (as far as I know because graphics card is an integrated one)

What do I want from the upgrade:
Bigger screen, bigger UI elements, better posture when I sit.
Performance upgrade.
To last me two years and stay fast with updated apps.


What do I need advice about:

4k or 5k?

i5 or i7 (to at least match my current i7)

How much GhZ for that processor?

SSD vs Fusion (to at least match my current 400 mbps read/write speeds)

Apple or 3rd party RAM?

Which graphics card? How many GB?


Thanks guys.

 

[maxmaut]

This just became super urgent and I really need your opinions guys.